Genetics: Principles of Heredity

Genetics

What is Heredity?

  • In Biology, heredity is the passing on of characteristics from one generation to the next. It is the reason why offspring look like their parents. It also explains why cats always give birth to kittens and never puppies. The process of heredity occurs among all living organisms, including animals, plants, bacteria, protists and fungi. Genetic variation refers to the variation in a population or species. Genetics is the study of heredity and variation in living organisms.
  • Two research approaches were historically important in helping investigators understand the biological basis of heredity. The first of these approaches, ‘transmission genetics’, involved crossing organisms and studying the offspring’s’ traits to develop hypotheses about the mechanisms of inheritance.
  • The second approach involved using cytological techniques to study the machinery and processes of cellular reproduction. This approach laid a solid foundation for the more conceptual understanding of inheritance that developed as a result of transmission genetics. Ever since the 1970’s, with the advent of molecular tools and techniques, geneticists are able to intensively analyze genetic basis of trait variation in various organisms, including plants, animal, and humans.

Genetics

Genetics is the science which deals with the mechanisms responsible for similarities and differences among closely related species. The term ‘genetic’ was coined by W.Batesmanin 1905. It is derived from the Greek word ‘genesis’ meaning grow into or to become. So, genetic is the study of heredity and hereditary variations it is the study of transmission of body features: ie, similarities and difference, from parents to offspring and the laws related to this transmission.

Variation

Any difference between individual organisms or groups of organisms of any species, caused either by genetic difference or by the effect of environmental factors, is called variation. Variation can be shown in physical appearance, metabolism, behavior, learning and mental ability, and other obvious characters.

Types of Variation

There are two types of variation

Genotypic variations: – Genotypic variations are caused by differences in the number or structure of chromosomes or by difference in the genes carried by the chromosome. Height, eye colour, body forms are some of the genotypic variations. A variation cannot be identified as genotype by simply observing the organism unless breeding experiments are performed under controlled environmental conditions.

Somatic variations: – Somatic variations may result from several factors, such as climate, food supply, and actions of other organisms. These variations are not due to differences in genes or chromosomes, and in general are not transmitted future generations. Hence they are not significance in the process of evolution.

Heredity: – Is defined as the transmission of characteristics from parents to offspring. It can also be defined as resemblances among individuals related to descent. It also means the inheritance of like qualities or characters from the one generation to the next and to successive generations.

Mendel’s Laws of Heredity

Gregor Johann Mendel (1822-1844) is known as the father of genetics as he was the first to demonstrate the mechanism of transmission of characters from one generation to the other. He carried out his work on garden pea, pisum sativum. He selected 7 pairs of contrasting traits of garden peas. Mendel’s gave three laws or principles of inheritance.

  1. Law of dominance: States that in heterozygous condition among two alleles of a character the alleles which expresses itself is dominant and the one which can’t express is recessive.
  1. Law of segregation: States that although the alleles of a character remain together for long time but they do not mix with each other and separate at the time of gametogenesis so that each gamete receives only one alleles of a character either dominant or recessive.
  1. Law of Independent assortment states that alleles of a character can undergo any sort of combination to give rise to a phenotype differing from both the parents.

Notations used in Breeding Experiments

The dominant trait is written with a capital letter .for example tallness is represented as T and darkness is represented with the corresponding small letters t. if tallness is due to both the dominant alleles, it is written as TT. If tallness is due to only one dominant trait then it is written as Tt. If both the alleles are receives, making the organism draft, then it is written as tt. A homozygous condition is one in which both alleles are of the same nature, for example .Tt or tt. Heterozygous condition (here, the two alleles are of different nature) is written as Tt. In a dihybrid cross, two characters are taken into account .Hence the notation for the homozygous dominant would be AABB, and for the homozygous receives it would be aabb. When the gamete is formed the traits are separated, as the chromosome number during meiosis is halved.

Law of Segregation

When the tall plants in F1 were crossed among themselves, the F2 generation and 75% tall plants and 25% dwarf plants (ratio 3:1) This led Mendel to conclude that the alleles representing darkness were intact and were neither lost nor contaminated .Medals study with one character (monohybrid cross)led to the formulation of the law or principle of segregation. This means that although the alleles of a character remain together, they are separated in subsequent generations.


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